The end-game for (A)D&D is something that many in the OSR are rediscovering, and it's something that I'll be placing some emphasis on in my own Emprise!™ game. However, in thinking about it, there are some interesting setting implications that the stated end-game rules pose that are possibly not all that apparent at first blush.
First off, a quick review. Clerics (at 9th level) can clear an area, attract followers, and get 9 s.p. per month per inhabitant. Fighters and rangers (also at 9th level) can clear an area, attract followers, and get 7 s.p. per inhabitant. Magic-users (at 12th level) can do the same, and get a 5 s.p. per person tax every month. Thieves (at 10th level) set up their headquarters in or near an existing town, and start their own guild, so their end-game experience is very different than other classes.
In those simple rules there are some interesting things unstated. First off, there is assumed to be land that can be "cleared". Enough that, following the rules on p. 93 of the DMG, areas of seven 30-mile hexes can be cleared as part of a single freehold. Thus, the campaign setting needs to have some borderlands that can be pushed out and expanded as civilization grows and more hexes are cleared of monsters. Where's this land? In the World of Greyhawk, for example, there don't seem to be many places where there's wilderness to expand into. Is it the areas "between" the kingdoms? If so, the DM has to make sure he's accounted for this, and that the kingdoms' claimed territory is much larger than the actual, populated and "tame" land.
There is also the prospect that there are people-- peasants, tradesmen, artisans, etc.-- in the campaign that are both free to move around (not a very historical thing in a medieval setting) and willing to go out to the new edge of the wilderness to settle in under the PC in the newly cleared region. Where are these folks coming from? What sort of social forces are at work that encourage the establishment of new hamlets and villages? What's the motivation for picking up and moving? Are the taxes under a PC going to be lower than those in the core of the realm? If so, the numbers cited above give some baselines for what to expect in the interior. And what are the existing landlords going to think of these periodic exoduses of taxpayers out to the edge of the wilderness? Are they trying to get rid of their malcontents by sending them out that way?
That brings up another interesting rule from the DMG, on p. 94. Apparently, those little silver-piece generators will go into open revolt no less than once every 5 years! And that's going to be 20% of the population, growing by 10% and getting more and more capable every month. Egads!